Ceramic materials are of critical importance for a number of high temperature, high performance applications such as gas turbines. These applications require a unique combination of properties such as high specific strength, high temperature mechanical property retention, low thermal and electrical conductivity, hardness and wear resistance, and chemical inertness. Design reliability and the need for economical fabrication of complex shapes, however, have prevented ceramic materials from fulfilling their potential in these critical high temperature, high performance applications.
The design reliability problems with ceramics, and the resultant failure under stress, are due largely to the relatively brittle nature of ceramics. This, in combination with the high cost of fabricating complex shapes, has limited the usage of ceramics.
To overcome the problems associated with forming ceramic materials into molded products, various alternatives have been suggested. For example, it is believed that ceramics made from organosilicon polymers have the potential to overcome the problems associated with molding and sintering inorganic ceramics. Thus, polymers based on silicon, carbon and/or nitrogen and oxygen have been developed. See for example, "Siloxanes, Silanes and Silazanes in the Preparation of Ceramics and Glasses" by Wills et al, and "Special Heat-Resisting Materials from Organometallic Polymers" by Yajima, in Ceramic Bulletin, Vol 62, No. 8, pages 893-915 (1983), and the references cited therein. Typically, the organosilicon preceramic polymers are pyrolyzed in an inert gas to form silicon carbide, silicon nitride-containing articles, especially fibers.
Another process for producing ceramic articles, including fibers, is disclosed in U.S. Pat. No. 3,399,979 and U.S. Pat. No. 3,403,008. According to these patents, a preformed organic polymeric material is impregnated with a solution of a metal compound, heated to leave a carbonaceous relic containing the metal in finally dispersed form and further heating at 1,000.degree.-2,000.degree. C. in a non-oxidizing atmosphere to form the metal carbide or metal nitride depending on the atmosphere utilized. A similar approach has been taken in the formation of metal oxide fibers. Thus, as disclosed in U.S. Pat. Nos. 3,846,527 and 4,010,233 metal salts are incorporated into polymeric spinning solutions, the solutions spun into fibers, and the fibers calcined in air to yield metal oxide fibers. Use of alternative calcination atmospheres leads to the formation of metal carbide or nitride fibers. Useful of metal salt mixtures are disclosed as resulting in bimetallic oxide fibers.
Still another approach has been to disperse ceramic powders in a carrier component such as organic liquids including low molecular weight polymers, spinning the dispersion into fibers and then sintering the ceramics. An example of this procedure for forming ceramic fibers such as ferrimagnetic spinel fibers is disclosed in U.S. Pat. No. 4,559,191.
U.S. Pat. No. 4,126,652 discloses a process for preparing metal carbide-containing molded products which comprises heating a molded composition comprising at least one powdery metal selected from the group consisting of B, Ti, Si, Zr, Hf, V, Nb, Ta, Mo, W, Cr, Fe, and U and having an average particle size of not more than 50 microns and an acrylonitrile polymer at a temperature of about 200.degree.-400.degree. C., and then calcining the resulting product at a temperature of about 900.degree.-2,500.degree. C. in an inert atmosphere to form the metal carbide. Metal carbide fibers can be formed by the process which involves spinning the mixture of metal and carbon-forming polymer into fiber, heating to render the fibers infusible and then pyrolyzing to yield the metal carbide. The metals may be added together with any conventional calcining acid including metal oxides. One example in the patent describes adding metallic tungsten and metallic silicon to a polyacrylonitrile solution and ultimately forming fiber consisting of tungsten carbide and silicon carbide.
The present invention is based in part, in an attempt to improve the process of this latter mentioned patent and to provide improved ceramic articles, including fibers. It is known that certain diverse ceramics when combined as an alloy or solid solution possess superior properties than either of the ceramic materials alone or mere mixtures thereof. For example, it has recently been found that an alloy of SiC and AlN in comparison with SiC possesses superior creep resistance, improved fracture toughness, lower thermal conductivity, and possibly enhanced oxidation and corrosion resistance. It is believed that AlN-SiC solid solutions or alloys will be an important class of structural ceramics. A method of forming AlN-SiC solid solutions other than by hot pressing the inorganic powder has been reported by Rafaniello et al wherein using AlCl.sub.3.6H.sub.2 O, starch and SiO.sub.2 fine powder as starting materials, and heating, a sintered powder comprising a SiC-AlN solid solution having improved properties relative to SiC was prepared (Journal of Materials Science 16 (1981) 3479-3488). An improved process for forming silicon carbide and aluminum nitride solid solutions is set forth in copending, commonly assigned application U.S. Ser. No. 872,312, filed June 9, 1986 wherein an organo-aluminum preceramic polymer containing a backbone comprised of alternating aluminum- and nitrogen-containing groups is mixed with an organosilicon preceramic polymer and pyrolyzed in an inert atmosphere.
Although the aforementioned U.S. Pat. No. 4,126,652 discloses numerous metals which can be added to a carbon-forming polymer to form metal carbides, and in at least one example illustrates the addition of two diverse metals to form a fiber comprising a mixture of the corresponding metal carbides, there is no specific mention or desirability of forming ceramic alloys or solid solutions by the method disclosed in the patent.
Another shortcoming associated with the process for forming metal carbides as disclosed in U.S. Pat. No. 4,126,652, is that many of the useful metal powders are refractory materials and are, thus, relatively inactive. Such refractory materials for example, are difficult to react with carbon to form metal carbides. Accordingly, carbonizing temperatures well above 1,000.degree. C. are needed to react the refractory metals with the carbonaceous polymer to form the metal carbide. The need for such high temperatures increases the difficulty and ultimate cost of performing the process.
It is one object of the present invention to provide a novel method for forming ceramic alloys or solid solutions.
Another important object of the present invention is to provide an improved method for forming metal carbides from dispersions of fine metallic particles in carbonaceous polymers by improving the reactivity of the metals, in particular, the refractory metals with the carbonaceous polymer.
These and other objects, aspects and advantages, as well as the scope, nature and utility of the present invention, will be apparent from the following description and appended claims.